C++11 use in Chromium
This document lives at src/styleguide/c++/c++11.html in a Chromium checkout.
This document summarizes the features of C++11 (both in the language itself and in enhancements to the Standard Library) and describes which features are allowed in Chromium and contains pointers to more detailed information. The Guide applies to Chromium and its subprojects. Subprojects can choose to be more restrictive if they need to compile on more toolchains than Chromium.
You can propose to make a feature available or to ban a
feature by sending an email to cxx@chromium.org.
Ideally include a short blurb on what the feature is, and why you think it
should or should not be allowed. Ideally, the list will arrive at some
consensus and the wiki page will be updated to mention that consensus. If
there's no consensus, src/styleguide/c++/OWNERS
get to decide --
for divisive features, we expect the decision to be to not use the feature yet
and possibly discuss it again a few months later, when we have more experience
with the language.
Table of Contents
C++11 Allowed Features
The following features are currently allowed.
Feature | Snippet | Description | Documentation Link | Notes and Discussion Thread |
---|---|---|---|---|
Aliases | using new_alias = typename |
Allow parameterized typedefs | Type alias (using syntax) | Use instead of typedef, unless the header needs to be compatible with C. Discussion thread |
Angle Bracket Parsing in Templates | >> for > > , <:: for < :: |
More intuitive parsing of template parameters | C++ Templates Angle Brackets Pitfall | Recommended to increase readability. Approved without discussion. |
Arrays | std::array |
A fixed-size replacement for built-in arrays, with STL support | std::array | Useful in performance-critical situations, with small, fixed-size arrays. In most cases, consider std::vector instead. std::vector is cheaper to std::move and is more widely used. Discussion thread. |
Automatic Types | auto |
Automatic type deduction | auto specifier | Google Style Guide. Discussion thread. Another discussion thread. |
Constant Expressions | constexpr |
Compile-time constant expressions | constexpr specifier | Prefer to const for variables where possible. Use cautiously on functions. Don't go out of the way to convert existing code. Google Style Guide |
Declared Type Accessor | decltype(expression) |
Provides a means to determine the type of an expression at compile-time, useful most often in templates. | decltype specifier | Usage should be rare. Discussion thread |
Default Function Creation | Function(arguments) = default; |
Instructs the compiler to generate a default version of the indicated function | What's the point in defaulting functions in C++11? | Discussion thread |
Default Function Template Arguments | template <typename T = type> |
Allow function templates, like classes, to have default arguments | Default Template Arguments for Function Templates | Discussion thread |
Delegated Constructors | Class() : Class(0) {} |
Allow overloaded constructors to use common initialization code | Introduction to the C++11 feature: delegating constructors | Discussion thread |
Enumerated Type Classes and Enum Bases | enum class classname |
Provide enums as full classes, with no implicit conversion to booleans or integers. Provide an explicit underlying type for enum classes and regular enums. | enum-class | Enum classes are still enums and follow enum naming rules (which means SHOUTY_CASE in the Chromium Style Guide). Discussion thread |
Explicit Conversion Operators | explicit operator type() { ... } |
Allows conversion operators that cannot be implicitly invoked | explicit specifier | Prefer to the "safe bool" idiom. Discussion thread |
Final Specifier | final |
Indicates that a class or function is final and cannot be overridden | final Language Reference | Recommended for new code. Discussion thread |
Function Suppression | Function(arguments) = delete; |
Suppresses the implementation of a function, especially a synthetic function such as a copy constructor | TODO: documentation link | Discussion thread |
Lambda Expressions | [captures](params) -> ret { body } |
Anonymous functions | Lambda functions | Do not bind or store lambdas outside the lifetime of the stack frame they are defined in; use base::Bind and base::Callback instead, because they offer protection against a large class of object lifetime mistakes. Don't use default captures ([=] , [&] – Google Style Guide). Lambdas are typically useful as a parameter to methods or functions that will use them immediately, such as those in <algorithm> . Discussion thread |
Local Types as Template Arguments | void func() { |
Allows local and unnamed types as template arguments | Local types, types without linkage and unnamed types as template arguments | Usage should be rare. Approved without discussion. |
Non-Static Class Member Initializers | class C {
| Allows non-static class members to be initialized at their definitions (outside constructors) | Non-static data members | Discussion thread |
Null Pointer Constant | nullptr |
Declares a type-safe null pointer | nullptr | Prefer over NULL or 0 . Discussion thread. Google Style Guide. std::nullptr_t can be used too. |
Override Specifier | override |
Indicates that a class or function overrides a base implementation | override Language Reference | Recommended for new code. Discussion thread |
Range-Based For Loops | for (type var : range) |
Facilitates a more concise syntax for iterating over the elements of a container (or a range of iterators) in a for loop |
Range-based for loop | As a rule of thumb, use for (const auto& ...) , for (auto& ...) , or for (concrete type ...) . For pointers, use for (auto* ...) to make clear that the copy of the loop variable is intended, and only a pointer is copied. Discussion thread |
Rvalue References | T(T&& t) and T& operator=(T&& t) |
Reference that only binds to a temporary object | Rvalue references | As per the Google Style Guide: Only use these to define move constructors and move assignment operators, and for perfect forwarding. Most classes should not be copyable, even if movable. Continue to use DISALLOW_COPY_AND_ASSIGN in most cases. Discussion thread. Another discussion thread. |
Standard Integers | Typedefs within <stdint.h> and <inttypes> |
Provides fixed-size integers independent of platforms | <stdint.h> (cstdint) | Already in common use in the codebase. Approved without discussion. |
Static Assertions | static_assert(bool, string) |
Tests compile-time conditions | Static Assertion | Discussion thread |
Trailing Return Types | auto function declaration -> return_type |
Allows trailing function return value syntax | Declaring functions | Use only where it considerably improves readability. Discussion thread. Another discussion thread. |
Uniform Initialization Syntax | type name {[value ..., value]}; |
Allows any object of primitive, aggregate or class type to be initialized using brace syntax | Uniform initialization syntax and semantics | See the Chromium C++ Dos And Don'ts guidance on when to use this. Discussion thread |
Variadic Macros | #define MACRO(...) Impl(args, __VA_ARGS__) |
Allows macros that accept a variable number of arguments | Are Variadic macros nonstandard? | Usage should be rare. Discussion thread |
Variadic Templates | template <typename ... arg> |
Allows templates that accept a variable number of arguments | Parameter pack | Usage should be rare. Use instead of .pump files. Discussion thread |
C++11 Allowed Library Features
The following library features are currently allowed.
Feature or Library | Snippet | Description | Documentation Link | Notes |
---|---|---|---|---|
Access to underlying std::vector data |
v.data() |
Returns a pointer to a std::vector 's underlying data, accounting for empty vectors. |
std::vector::data | Discussion thread |
Algorithms | All C++11 features in <algorithm> :all_of , any_of , none_of find_if_not copy_if , copy_n move , move_backward (see note)shuffle is_partitioned , partition_copy , partition_point is_sorted , is_sorted_until is_heap , is_heap_until minmax , minmax_element is_permutation |
Safe and performant implementations of common algorithms | <algorithm> | Discussion thread Note that <algorithm> contains a range-based move method. This is allowed, but because people may confuse it with the single-arg std::move , there is often a way to write code without it that is more readable. Discussion thread |
Begin and End Non-Member Functions | std::begin() , std::end() |
Allows use of free functions on any container, including fixed-size arrays | std::begin, std::end | Useful for fixed-size arrays. Note that non-member cbegin() and cend() are not available until C++14. Discussion thread |
Conditional Type Selection | std::enable_if , std::conditional |
Enables compile-time conditional type selection | std::enable_if, conditional | Usage should be rare. Discussion thread |
Constant Iterator Methods on Containers | E.g. std::vector::cbegin() , std::vector::cend() |
Allows more widespread use of const_iterator |
std::vector::cbegin, std::vector::cend | Applies to all containers, not just vector . Consider using const_iterator over iterator where possible for the same reason as using const variables and functions where possible; see Effective Modern C++ item 13 for motivation. Discussion thread |
Containers containing movable types | vector<scoped_ptr> |
Enables containers that contain move-only types like scoped_ptr |
TODO | Prefer over ScopedVector. |
Declared Type As Value | std::declval<class>() |
Converts a type to a reference of the type to allow use of members of the type without constructing it in templates. | std::declval | Usage should be rare. Discussion thread |
Emplacement methods for containers | emplace() , emplace_back() , emplace_front() , emplace_hint() |
Constructs elements directly within a container without a copy or a move. Less verbose than push_back() due to not naming the type being constructed. |
E.g. std::vector::emplace_back | std::map::emplace() is not yet available on all libstdc++ versions we support. When using emplacement for performance reasons, your type should probably be movable (since e.g. a vector of it might be resized); given a movable type, then, consider whether you really need to avoid the move done by push_back() . For readability concerns, treat like auto ; sometimes the brevity over push_back() is a win, sometimes a loss. Discussion thread |
Forwarding references | std::forward() |
Perfectly forwards arguments (including rvalues) | std::forward | Allowed, though usage should be rare (primarily for forwarding constructor arguments, or in carefully reviewed library code). Discussion thread |
Initializer Lists | std::initializer_list<T> |
Allows containers to be initialized with aggregate elements | std::initializer_list | Be cautious adding new constructors which take these, as they can hide non-initializer_list constructors in undesirable ways; see Effective Modern C++ Item 7 for more. |
Math functions | All C++11 features in <cmath> , e.g.:INFINITY , NAN , FP_NAN float_t , double_t fmax , fmin , trunc , round isinf , isnan |
Useful for math-related code | <cmath> | Discussion thread |
Move Iterator Adaptor | std::make_move_iterator() |
Wraps an iterator so that it moves objects instead of copying them. | std::make_move_iterator | Useful to move objects between containers that contain move-only types like scoped_ptr . Discussion thread |
Move Semantics | std::move() |
Facilitates efficient move operations | std::move | Discussion thread |
String Direct Reference Functions | std::string::front() , std::string::back() |
Returns a reference to the front or back of a string | std::basic_string::front, std::basic_string::back | Discussion thread |
Type Traits | All C++11 features in <type_traits> except for aligned storage (see separate item), e.g.:integral_constant is_floating_point , is_rvalue_reference , is_scalar is_const , is_pod , is_unsigned is_default_constructible , is_move_constructible , is_copy_assignable enable_if , conditional , result_of |
Allows compile-time inspection of the properties of types | <type_traits> | Note that not all type traits are available on all platforms (e.g. std::underlying_type doesn't work in libstdc++4.6). Use judiciously. Discussion thread |
Tuples | All C++11 features in <tuple> , e.g. std::tie and std::tuple . |
A fixed-size ordered collection of values of mixed types | <tuple> | Use base::get instead of std::get in case the std::tuple may be a rvalue-reference.
std::get in <=libstdc++-4.6 misses an overload for rvalue-reference of a tuple, and base::get complements it. |
Unordered Associative Containers | std::unordered_set , std::unordered_map , std::unordered_multiset , std::unordered_multimap |
Allows efficient containers of key/value pairs | std::unordered_map, std::unordered_set | Per the Google Style Guide, specify custom hashers instead of specializing std::hash for custom types. Discussion thread. |
Unique Pointers | std::unique_ptr<type> |
A smart pointer with sole ownership of the owned object. | std::unique_ptr | Use in all newly written code. Google Style Guide. Discussion thread. scoped_ptr is a typedef for std::unique_ptr and is going away shortly. |
C++11 Blacklist (Disallowed and Banned Features)
This section lists features that are not allowed to be used yet.
C++11 Banned Features
This section lists C++11 features that are not allowed in the Chromium codebase.
Feature or Library | Snippet | Description | Documentation Link | Notes |
---|---|---|---|---|
Alignment Features | alignas specifier, alignof operator |
Object alignment | alignas, alignof | Reevaluate now that MSVS2015 is available. Discussion thread |
Function Local Variable | __func__ |
Provides a local variable of the name of the enclosing function for logging purposes | The __func__ Predeclared Identifier is Coming to C++ | Reevaluate now that MSVS2015 is available. Discussion thread |
Inherited Constructors | class Derived : Base { |
Allow derived classes to inherit constructors from base classes | Using-declaration | Reevaluate now that MSVS2015 is available. Discussion thread |
long long Type |
long long var= value; |
An integer of at least 64 bits | Fundamental types | Use a stdint.h type if you need a 64bit number. Discussion thread |
Raw String Literals | string var=R"(raw_string)"; |
Allows a string to be encoded without any escape sequences, easing parsing in regex expressions, for example | string literal | Causes incorrect line numbers in MSVS2013 and gcc. Reevaluate once that works. Discussion thread |
UTF-16 and UTF-32 Support (16-Bit and 32-Bit Character Types) | char16_t and char32_t |
Provides character types for handling 16-bit and 32-bit code units (useful for encoding UTF-16 and UTF-32 string data) | Fundamental types | Reevaluate now that MSVS2015 is available. Non-UTF-8 text is banned by the Google Style Guide. However, may be useful for consuming non-ASCII data. Discussion thread |
UTF-8, UTF-16, UTF-32 String Literals | u8"string", u"string", U"string" |
Enforces UTF-8, UTF-16, UTF-32 encoding on all string literals | string literal | Reevaluate now that MSVS2015 is available. Discussion thread |
Ref-qualified Member Functions | class T { |
Allows class member functions to only bind to |this| as an rvalue or lvalue. | Member functions | Banned in the Google Style Guide. Banned in Chromium except by explicit approval from styleguide/c++/OWNERS . Discussion Thread |
C++11 Banned Library Features
This section lists C++11 library features that are not allowed in the Chromium codebase.
Feature | Snippet | Description | Documentation Link | Notes |
---|---|---|---|---|
Chrono Library | <chrono> |
Provides a standard date and time library | Date and time utilities | Duplicated Time APIs in base/ . Keep using the base/ classes. |
Regex Library | <regex> |
Provides a standard regular expressions library | Regular expressions library | We already have too many regular expression libraries in Chromium. Use re2 when in doubt. |
Thread Library | <thread> support, including<future> , <mutex> , <condition_variable> |
Provides a standard mulitthreading library using std::thread and associates |
Thread support library | C++11 has all kinds of classes for threads, mutexes, etc. Since we already have good code for this in base/ , we should keep using the base classes, at least at first. base::Thread is tightly coupled to MessageLoop which would make it hard to replace. We should investigate using standard mutexes, or unique_lock, etc. to replace our locking/synchronization classes. |
Atomics | std::atomic and others in <atomic> |
Fine-grained atomic types and operations | <atomic> | Use in tcmalloc has caused performance regressions. Banned until we understand this better. Discussion Thread |
Shared Pointers | std::shared_ptr |
Allows shared ownership of a pointer through reference counts | std::shared_ptr | Needs a lot more evaluation for Chromium, and there isn't enough of a push for this feature. Google Style Guide. Discussion Thread. |
C++11 Features To Be Discussed
The following C++ language features are currently disallowed. See the top of this page on how to propose moving a feature from this list into the allowed or banned sections. Note that not all of these features work in all our compilers yet.
Feature | Snippet | Description | Documentation Link | Notes |
---|---|---|---|---|
Attributes | [[attribute_name]] |
Attaches properties to declarations that specific compiler implementations may use. | C++11 generalized attributes | |
Exception Features | noexcept , exception_ptr , current_exception() , rethrow_exception , nested_exception |
Enhancements to exception throwing and handling | std::exception | Exceptions are banned by the Google Style Guide. Discussion thread |
Inline Namespaces | inline |
Allows better versioning of namespaces | Namespaces | Unclear how it will work with components |
Union Class Members | union name {class var} |
Allows class type members | Union declarations | |
User-Defined Literals | type var = literal_value_type |
Allows user-defined literal expressions | User-defined literals |
C++11 Standard Library Features To Be Discussed
The following C++ library features are currently disallowed. See the top of this page on how to propose moving a feature from this list into the allowed or banned sections. Note that not all of these features work in all our compilers yet.
Feature | Snippet | Description | Documentation Link | Style Guide Usage |
---|---|---|---|---|
Address Retrieval | std::addressof() |
Obtains the address of an object even with overloaded operator& |
std::addressof | Usage should be rare as operator overloading is rare and & should suffice in most cases. May be preferable over & for performing object identity checks. |
Aligned Storage | std::aligned_storage<Size, Align>::type std::alignment_of<T> , std::aligned_union<Size, ...Types> std::max_align_t |
Declare uninitialized storage having a specified alignment, or determine alignments. | std::aligned_storage | std::aligned_storage and std::aligned_union are disallowed in google3 over concerns about compatibility with internal cross-compiling toolchains. |
Allocator Traits | std::allocator_traits |
Provides an interface for accessing custom allocators | std::allocator_traits | Usage should be rare. |
Bind Operations | std::bind(function, args, ...) |
Declares a function object bound to certain arguments | TODO: documentation link | |
C Floating-Point Environment | <cfenv> , <fenv.h> |
Provides floating point status flags and control modes for C-compatible code | Standard library header <cfenv> | Banned in Google Style Guide due to concerns about compiler support. |
Complex Inverse Trigonometric and Hyperbolic Functions | Functions within <complex> |
Adds inverse trigonomentric and hyperbolic non-member functions to the <complex> library. |
std::complex | |
Container Compaction Functions | std::vector::shrink_to_fit() , std::deque::shrink_to_fit() , std::string::shrink_to_fit() |
Requests the removal of unused space in the container | std::vector::shrink_to_fit, std::deque::shrink_to_fit, std::basic_string::shrink_to_fit | |
Date/Time String Formatting Specifiers | std::strftime() |
Converts date and time information into a formatted string using new specifiers | std::strftime | |
Function Return Type Deduction | std::result_of<Functor(ArgTypes...)> |
Extracts the return type from the type signature of a function call invocation at compile-time. | std::result_of | Why does std::result_of take an (unrelated) function type as a type argument? |
Function Objects | std::function |
Wraps a standard polymorphic function | TODO: documentation link | |
Forward Lists | std::forward_list |
Provides an efficient singly linked list | std::forward_list | |
Gamma Natural Log | std::lgamma() |
Computes the natural log of the gamma of a floating point value | std::lgamma | |
Garbage Collection Features | std::{un}declare_reachable() , std::{un}declare_no_pointers() |
Enables garbage collection implementations | std::declare_reachable, std::declare_no_pointers | |
Iterator Operators | std::next() , std::prev() |
Copies an iterator and increments or decrements the copy by some value | std::next, std::prev | |
Pointer Traits Class Template | std::pointer_traits |
Provides a standard way to access properties of pointers and pointer-like types | std::pointer_traits | Useful for determining the element type pointed at by a (possibly smart) pointer. |
Random Number Generators | Functions within <random> |
Random number generation algorithms and utilities | Pseudo-random number generation | |
Ratio Template Class | std::ratio<numerator, denominator> |
Provides compile-time rational numbers | std::ratio | Banned in Google Style Guide due to concerns that they are tied to a more template-heavy interface style. |
Reference Wrapper Classes | std::reference_wrapper and std::ref() , std::cref() |
Allows you to wrap a reference within a standard object (and use those within containers) | Reference Wrappers | |
Soft Program Exits | std::at_quick_exit() , std::quick_exit() |
Allows registration of functions to be called upon exit, allowing cleaner program exit than abort() or exit |
std:quick_exit | |
String to Number Functions | std::stoi() , std::stol() , std::stoul() , std::stoll , std::stoull() , std::stof() , std::stod() , std::stold() , std::to_string() |
Converts strings to numbers | std::stoi, std::stol, std::stoll, std::stoul, std::stoull, std::stof, std::stod, std::stold | |
System Errors | <system_error> |
Provides a standard system error library | std::system_error | |
Type-Generic Math Functions | Functions within <ctgmath> |
Provides a means to call real or complex functions based on the type of arguments | Standard library header <ctgmath> | |
Type Info Enhancements | std::type_index , std::type_info::hash_code() |
Allows type information (most often within containers) that can be copied, assigned, or hashed | std::type_index, std::type_info::hash_code | std::type_index is a thin wrapper for std::type_info , allowing you to use it directly within both associative and unordered containers |
Variadic Copy Macro | va_copy(va_list dest, va_list src) |
Makes a copy of the variadic function arguments | ||
Weak Pointers | std::weak_ptr |
Allows a weak reference to a std::shared_ptr |
std::weak_ptr | Ownership and Smart Pointers |
Wide String Support | std::wstring_convert , std::wbuffer_convert std::codecvt_utf8 , std::codecvt_utf16 , std::codecvt_utf8_utf16 |
Converts between string encodings | std::wstring_convert, std::wbuffer_convert, std::codecvt_utf8, std::codecvt_utf16, std::codecvt_utf8_utf16 | Non-UTF-8 text is banned by the Google Style Guide. However, may be useful for consuming non-ASCII data. |